NanoHost

I prefer this to Savihost as it is faster and more reliable, it does have it's limitation though, the most apparent it doesn't do VST3, but for just trying out a synth it's good enough. For 32-bit synths I would recommend the Minihost though.

If you're looking for something which can host more than one at a time I recommend the free version of Voltage Modular and use the stand alone as an VST host. Set up a preset with as many VST host modules as you want. It does VST2 and VST3, only 64-bit though.